1. Field of the Invention
The present invention relates generally to the field of tissue injury analysis. More particularly, the present invention relates to a method and apparatus for objective tissue injury analysis. Specifically, a preferred embodiment of the present invention relates to the use of a fiber optic probe, a spectrometer and a hybrid neural network to increase the accuracy of tissue injury analysis. The present invention thus relates to a method and apparatus for tissue injury analysis of the type that can be termed objective.
2. Discussion of the Related Art
Within this application several publications are referenced by. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the present invention and illustrating the state of the art.
Tissue injury is common in daily life. For example, approximately 70,000 serious burn cases are reported in the United States every year, at a cost to the economy of an estimated two billion dollars. Traditionally, burns have been classified as first, second, or third degree, based on visual criteria. First degree burns are visually indicated by redness and blistering of the skin. Second degree burns are visually indicated by withering of the skin without charring. Third degree burns are visually indicated by eschar formation and charring.
This type of classification, which has been used with only minor alterations for nearly two hundred years, is concerned chiefly with the intensity of burning and not with the depth of tissue destroyed. Only recently have burn physicians come to realize that the depth of injury is of greater importance than superficial appearance. The classification of burns that has recently been adopted has completely forsaken all reference to outward appearances, which are only an indication of the severity of surface burning. The new type of classification recognizes two degrees of burn injury. The first new classification is partial thickness skin loss, implying the presence of sufficient living epithelial elements to resurface the area. The second new classification is full-thickness skin loss, implying virtually complete destruction of all epithelial elements so that healing can only occur by contraction of the wound and epithelial cell migration from the edge of the wound or by surgical intervention.
Proper treatment depends on the correct classification of the burn. Further, early differentiation between these two degrees of burns is critical for several reasons. It is better to excise dead tissue and close the wound than to allow spontaneous separation of the slough, with its attendant risks of infection, fibrosis, and loss of function. Surgical results are best when the proper treatment is taken within the shortest time. The sooner a definite diagnosis is made, the sooner the patient with partial-thickness burns can leave the hospital, decreasing costs for both the hospital and the patient. In life-threatening burns, when donor sites are at a premium, it is very important to distinguish quickly between full-thickness and partial-thickness burn injuries.
FIG. 1 shows a model of a three dimensional section of human skin. Two major tissue layers are conventionally recognized as constituting human skin 5. The outer layer is a thin stratified epithelium, called the epidermis 10, which varies relatively little in thickness over most of the body. The human epidermis is typically between 75 .mu.m and 150 .mu.m thick. Underlying the epidermis 10 is a dense layer of fibrous elastic connective tissue, called the dermis 20, which constitutes the mass of skin. The dermis 20 supports extensive vascular and nerve networks, and encloses specialized excretory and secretory glands and keratinized appendage structures such as hair and nail. Beneath the skin is the subcutaneous tissue, or hypodermis 50, which is variously composed of loose areolar connective tissue or fatty connective tissue displaying substantial regional variations in thickness. Nerves 25 pass through the hypodermis 50. Of particular interest is the presence and depth of hair follicles 30 and sweat glands 40 in the dermis. The bases of these structures are surrounded by cells capable of forming new "skin." These cells lie very close to the interface of the dermis and the subcutaneous fat 60, and represent the vital plane insofar as spontaneous skin repair is concerned. If destruction occurs below this vital plane, the burn is a full-thickness burn; if above this vital plane, it is a partial-thickness burn.
The blood supply in the skin comes from cutaneous branches of the subcutaneous musculocutaneous arteries. Branches arising from the cutaneous arteries give rise to a distinctive small vessel plexus which lies deep in the dermis near and parallel to the interface with the subcutaneous tissue. Therefore, destruction of a large area of hair follicles and sweat glands in a full-thickness burn devascularizes the skin in the same area. This is the basis of several previous burn diagnosis methods that use the new type of classification.
However, classifying a burn is not easy immediately after the burn occurs, and usually depends upon intuition about the appearance of the burn rather than upon accurate description and definition (i.e., objective characterization). Early visual assessment may be difficult because the ability of the wound to heal depends strongly on the condition of underlying tissues, which in the case of severe burns are generally obscured by overlying layers of dead and denatured skin. Thus, three days after burns were incurred, the surgeons in one study were only willing to commit themselves to a predication in about two thirds of the cases. Heimbach, D. M., Afromowitz, M. A., Engrav, L. H., Marvin, J. A. and Perry, B., "Burn Depth Estimation: Man or Machine," The Journal of Trauma, vol. 24, No. 5, pp. 373-378 (1984). One fourth of the predictions made at this time were incorrect. In an effort to address this problem many objective diagnostic methods have been proposed by previous researches. These methods take information from the surface, as well as beneath the skin, and depend on the following criteria and procedures. One method depends on a fluorescein test for the presence of dermal circulation. Another method depends on staining reactions on the surface of the burn. Another method depends on sensitivity of the burn to pinprick. Yet another method depends on temperature variations within the burn area as evidenced by thermogram.
Although some progress has been made in laboratory testing, heretofore, no method has gained widespread clinical acceptance. The limitations of previous methods include poor burn depth predictive values on various selected days post-burn, excessive cost, cumbersome techniques, time-consuming techniques and techniques that often include a toxic reaction.
These previous methods can be classified either as invasive or non-invasive. The invasive methods include the fluorescence test, staining appearance and sensitivity to pinprick. The non-invasive approaches are the thermogram imaging and multispectral photographic analysis.
The fluorescence method employs a fluorometer to quantify fluorescence as a measure of burn depth. However, the fluorescein injected into the femoral vein in this method causes a toxic reaction in some patients. Green, H. A., Bua, D., Anderson, R. R. and Nishioka, N. S., "Burn Depth Estimation Using Indocyanine Green Fluorescence." Arch Dermatol, vol. 128, January, pp. 43-49 (1992).
The staining reaction method introduced by Patey and Scarff maps out areas of surface necrosis using dyes such as hematoxylin, marking the absence of blood circulation. Patey, D. H. and Scarff, R. W., British Journal of Surgeon, vol. 32, pp. 32 (1944). However, this method reveals nothing about skin layers deeper than the eye can see, whereas the critical layer in burns under the new type of classification is the deepest plane of living epithelial elements.
While the pin-prick method is self-explanatory, it is often inaccurate in predicting the depth of a burn. In addition, this method can result in significant blood loss. Jackson, D. M., "In Search of an Acceptable Burn Classification." British Journal of Plastic Surgeon, vol. 23, pp. 118-145 (1970).
Thermography, the measurement of the infrared waves emitted by all objects, is time consuming in that it usually requires at least 20 minutes in an air-conditioned room. Mladick, R., Georgiade, N. and Thorne, F., "A Clinical Evaluation of the Use of Thermography in Determining Degree of Burn Injury." Plastic and Reconstructive Surgery, Vol. 38, No. 6, pp. 512-518 (1966). Further, thermography devices are very costly.
Anselmo and Zawacki developed a method based on rationing the magnitudes of visible and infrared radiation from several spectral ranges. Anselmo, V. J. and Zawacki, B. E., "Multispectral Photographic Analysis: A new Quantitative Tool to Assist in the Early Diagnosis of Thermal Burn Depth." Annals of Biomedical Engineering, Vol. 5, pp. 179-193 (1977). Although their results were promising, the analysis time was too slow for clinical decision making.
Heimbach developed a burn depth estimation approach called the Burn Depth Indicator (BDI) method. Heimbach, D. M., Afromowitz, M. A., Engrav, L. H., Marvin, J. A. and Perry, B., "Burn Depth Estimation: Man or Machine," The Journal of Trauma, vol. 24, No. 5, pp. 373-378 (1984); Lu, T., Lerner, J., "Spectroscopy and Hybrid Neural Networks," to appear in the Proceedings of the IEEE, April, 1996; Lerner, J. M., Lu, M. Angel and K. Kyle, Enhancing Minimum Detection Levels of Chlorinated Hydrocarbons: One Example of the Power of Neural Net Assisted Spectroscopy, American Laboratory, September, 1993. It is similar to the method of Anselmo and Zawacki (relating burn depth to the ratios of red/green, red/infrared, and green/infrared light reflected from the burn wound), but is much faster. This approach is based on the premise that the reflectance intensity of different optical wavelength ranges corresponds to different degrees of burns, and more specifically on the premise that green and red light are important for partial-thickness burns and red and infrared are important for full-thickness burns. Heimbach's experimental results show that the BDI method is significantly more accurate than clinical assessment in cases where surgeons subjectively predicted burn injuries would not heal. The BDI method is reported to have maintained an accuracy of 79% for wounds for which the surgeons would not make a prediction.
However, limited data analysis techniques allowed Heimbach to choose only the average intensity in each of several specific frequency ranges. This may have restricted the prediction accuracy and the application of the BDI method because the details of these frequency ranges may be different for different degrees of burn even though their averages are nearly the same. Further, other frequency ranges may also contain information about the classification of a burn injury.
Other tissue injuries for which better assessment accuracy is needed include contusions, bed sores and subdural hematoma and skin cancer. Other areas in which improved assessment accuracy is needed include monitoring skin for signs of skin cancer and characterizing biological tissues in general for blood perfusion, oxygenation and arterial blood gas levels.